Driver circuit

ABSTRACT

A stepping motor includes two coils. A driver circuit drives the stepping motor by setting dissimilar phases of supply currents to these two coils. One terminal of one coil is connected to ground and another terminal is set to a high impedance state, and an induced voltage generated at that coil is detected as a voltage with respect to ground. Then, in accordance with the state of the detected induced voltage, the magnitude of motor drive current supplied to the two coils is controlled.

CROSS-REFERENCE TO RELATED APPLICATIONS

The entire disclosure of Japanese Patent Application Nos. 2009-217477,2009-217478 and 2009-217479 including specification, claims, drawings,and abstract is incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a driver circuit for a stepping motor,which includes two coils and rotates a rotor driven by the coils by thesetting of dissimilar phases of supply currents to these two coils.

2. Background Art

Among the various types of motors available, one representative type ofmotor capable of precisely determining position is a stepping motor.Stepping motors are widely utilized in various apparatuses, for example,in focusing and anti-shake mechanisms in cameras and in paper feedmechanisms in office automation equipment.

The stepping motor is generally driven by changing the rotating positionof the rotor by a current phase to two stator coils. Therefore, if therotor is rotated in accordance with the phase of current to the coils,the rotor rotates a predetermined amount regardless of the amount ofcurrent to the coils. Accordingly, the amount of current to the coils isgenerally set sufficiently large so that the rotor can rotate reliably.

There are demands to set the power consumption in electric equipment aslow as possible. These demands are particularly high in officeautomation equipment requiring high current or battery driven portableequipment. On the other hand, in the drive of stepping motors, settingthe amount of current to a magnitude at which the rotor rotates reliablymeans extra current flows to the coils and extra power is consumed.Furthermore, motor drive at high power causes irregular rotor rotationand also causes vibration, noise, and heat generation.

SUMMARY

The present invention detects an induced voltage and controls a motordrive current in accordance with the induced voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an overall configuration of a system including a drivercircuit and a motor.

FIG. 2 shows a configuration of the output circuit.

FIG. 3 shows a configuration of the drive current adjustment circuit.

FIG. 4 shows outputs and control states of the output circuit.

FIG. 5 shows relationships between drive current state and drive voltagewaveform.

FIG. 6 shows states of the induced voltage waveform.

FIG. 7 shows an estimated zero cross point.

DETAILED DESCRIPTION

Embodiments of the present invention will be described hereinafter withreference to the attached drawings.

Overall Configuration

FIG. 1 shows an overall configuration where a system is composed of adriver 10 and a motor 200. An input signal is input by the driver 100and the driver 100 supplies a drive current to the motor 200 inaccordance with the input signal. As a result, rotation of the motor 200is controlled in accordance with the input signal.

Here, the driver 100 includes an output control circuit 12 and the inputsignal is supplied to the output control circuit 12. The output controlcircuit 12 determines the drive waveform (phase) at a predeterminedfrequency in accordance with the input signal and determines theamplitude of the drive current by PWM control to create a drive controlsignal. Then, the created drive control signal is supplied to an outputcircuit 14.

The output circuit 14 is composed of a plurality of transistors, theswitching of which controls current from a power supply and generatesmotor drive currents, which are supplied to the motor 200.

The motor 200 is a stepping motor and has two coils 22 and 24 and arotor 26. The two coils 22 and 24 are arranged so as to be positionallydisplaced at an electrical angle of 90° to each other. Therefore, thedirection of the magnetic fields with respect to the rotor 26 at therotor central angle is also displaced at an electrical angle of 90° toeach other. Furthermore, the rotor 26 includes a permanent magnet, forexample, and its position where it is stable is determined in accordancewith the magnetic field from the two coils 22 and 24. Namely, regardingthe angle of rotation of the rotor, by supplying alternating currentshaving a phase difference of 90° to each other to the two coils arrangedat positions displaced by 90°, the current phases make it possible tomove and rotate the rotor 26. Furthermore, at the timing of a specificcurrent phase, stopping the change in current phase makes it possible tostop the rotor at a position in accordance with the current phase atthat time. In this manner, the rotation of the motor 200 is controlled.

Voltages of outputs OUT1 to OUT4 of four current paths to the two coils22 and 24 are supplied to a drive current adjustment circuit 30. Thedrive current adjustment circuit 30 determines current amplitude to themotor 200 on the basis of the voltages of outputs OUT1 to OUT4. Then, anadjustment signal for this current amplitude is supplied to the outputcontrol circuit 12. Therefore, the output control circuit 12 generatesthe drive control signal from the input signal and the adjustmentsignal.

Output Circuit Configuration

FIG. 2 shows a configuration of part of the output circuit 14 and onecoil 22 (24) of the motor 200.

In this manner, an arm composed of two transistors Q1 and Q2 connectedin series and an arm composed of two transistors Q3 and Q4 connected inseries are provided between the power supply and ground and the coil 22(24) is connected to a midpoint between the transistors Q1 and Q2 and toa midpoint between the transistors Q3 and Q4. Then, by turning on thetransistors Q1 and Q4 and turning off the transistors Q2 and Q3, acurrent flows in one direction to the coil 22 (24), and by turning offthe transistors Q1 and Q4 and turning on the transistors Q2 and Q3, acurrent flows in the opposite direction to the coil 22 (24) so as todrive the coils 22 and 24.

Providing two of these circuits enables the currents supplied to the twocoils 22 and 24 to be controlled individually.

Configuration of Drive Current Adjustment Circuit

An example configuration of the drive current adjustment circuit 30 isshown in FIG. 3. The voltages of OUT1 to OUT4 are input by an ADC 34 viathe four switches 32, respectively. The ADC 34 converts a voltageselected by the switches 32 and sequentially outputs a digital signal.The output of the ADC 34 is supplied to a control logic 36. The controllogic 36 determines the current amplitude to the motor 200 on the basisof the supplied voltage waveforms of OUT1 to OUT4 and supplies anadjustment signal for current amplitude to the output control circuit12.

The output control circuit 12 creates the drive control signal in PWMcontrol in accordance with the adjustment signal. Here, in the PWMcontrol system, there are the direct PWM control system and theconstant-current chopping system.

In the case of the direct PWM control system, PWM control is performedassuming the rectangular wave duty ratio and the current output areproportional. At this time, when an induced voltage develops at themotor, the actual current output value decreases. In the direct PWMcontrol system, a current output value can be adjusted by controllingthe rectangular wave duty ratio, which is to be a target, and acoefficient for adjusting the amplitude of the rectangular wave.

In the case of the constant-current chopping system, by detectingcurrent flowing through a resistor Rt, current for driving the motor isdetected and control is performed by varying the pulse width of therectangular wave so that the current becomes the target value. In theconstant-current chopping system, the current output value can beadjusted by varying the above-mentioned target value.

A driver circuit employing the direct PWM control system in theembodiment will be described.

Here, in the embodiment, the output voltages OUT1 to OUT4 to the fourcoil terminals are directly converted from analog to digital by the ADC34.

For this reason, a timing circuit 38 is included. On the basis of drivephase of each coil, the timing circuit 38 controls switching of theswitches 32 and controls switching of the transistors Q2 and Q4 in theoutput circuit 14. Namely, for coil 22 (24), one OUT terminal isconnected to ground and another OUT terminal is open. As a result, aninduced voltage appears at the OUT terminal on the open side. This isinput by the ADC 34 and the ADC 34 outputs a digital value indicatingamplitude.

Here, as described above, the output circuit for one coil 22 (24) has aconfiguration shown in FIG. 2. Then, the drive for one coil 22 (24)repeats a state where the transistor Q1 is PWM controlled while thetransistor Q4 is on and a state where the transistor Q2 is turned on andthe transistor Q3 is PWM controlled.

FIG. 4 shows a voltage waveform between OUT1 and OUT2, which is a drivevoltage applied to the coil 22, and a voltage waveform between OUT3 andOUT4, which is a drive voltage applied to the coil 24. In this manner,the drive waveforms to the two coils 22 and 24 differ in phase by 90°and the drive waveform of the coil 22 leads 90° compared to the drivewaveform of the coil 24.

Then, in the example of the voltage waveform between OUT3 and OUT4 inFIG. 2, when transitioning from the state where the transistor Q4 isturned on and the transistor Q1 is PWM controlled to the state where thetransistor Q2 is turned on and the transistor Q3 is PWM controlled,namely, in a step of 180° of the drive waveform, and when transitioningfrom the state where the transistor Q2 is turned on and the transistorQ3 is PWM controlled to the state where the transistor Q4 is turned onand the transistor Q1 is PWM controlled, namely, in a step of 0° of thedrive waveform, the induced voltage is detected.

Namely, in this period, with the transistors Q1 and Q3 off, thetransistor Q2 (or Q4) to be turned on in the next phase is turned on. Itshould be noted that the transistor Q4 (or Q2) is set to remain off.

In the example of FIG. 4, in the vicinity of electrical angle 0°,OUT1-OUT2 for the coil 22 is in the state where the transistor Q4 isturned on and the transistor Q1 is PWM controlled, and in the step ofelectrical angle 90°, the transistor Q2 is turned on and OUT1 connectsto ground GND, the transistors Q1, Q3, and Q4 are turned off and OUT2 isset to an open state. As a result, the induced voltage at the coil 22 isobtained at OUT2 and by turning on switch 32-2 the induced voltage isinput by the ADC 34. In the step of electrical angle 270°, thetransistor Q4 is turned on and OUT2 connects to ground GND, and thetransistors Q1, Q2, and Q3 turn off and OUT1 is set to an open state. Asa result, the induced voltage at the coil 22 is obtained at OUT1 and byturning on switch 32-1 the induced voltage is input by the ADC 34. Sincethe phase of the coil 24 is delayed 90°, OUT3 becomes open at electricalangle 0°, OUT4 connects to ground, switch 32-3 turns on, and the inducedvoltage of OUT3 is supplied to the ADC 34, and at electrical angle 180°,OUT4 becomes open, OUT3 connects to ground, switch 32-4 turns on, andthe induced voltage of OUT4 is supplied to the ADC 34.

The switching of the transistors Q1 to Q4 and the control of theswitches 32 in the output circuit 14 for the coils 22 and 24 for inducedvoltage measurement are performed by the timing circuit 38 on the basisof switching phase signal from the output control circuit 12.

The induced voltage of the coil 22 (24) is obtained as a difference ofboth terminal voltages. However, in the embodiment, since one terminalof the coil 22 (24) is connected to ground when the induced voltage ismeasured, at the other terminal in the open state, a value of thevoltage difference of both terminals of the coil 22 (24) is directlyobtained. Therefore, it is not necessary to detect the voltagedifference of both terminals of the coil with an op-amp and thecircuitry becomes simple. Furthermore, the OUT on the open side is aterminal on the side where the induced voltage rises and the input tothe ADC 34 is basically a positive voltage that can be directlyconverted into a digital signal at the ADC 34.

In this manner the induced voltage at a phase where the drive currentwaveform becomes 0 is sequentially detected by the ADC 34. Therefore, atthe two coils 22 and 24, detection is performed four times in oneelectrical angle period of the motor. The detection period of theinduced voltage is ⅛ period in the 1-2 phase excitation mode and 1/16period in the W1-2 phase excitation mode employed in the embodiment.

Next, FIG. 5 shows three examples of the drive voltage waveform and theinduced voltage waveform in the one coil 22. The induced voltagewaveform has a tendency of having the phase lead when the drive currentis high. When the drive current is optimum, the phases of the drivevoltage waveform and the induced voltage waveform substantially match.On the other hand, when the drive current is low, the drive of the rotoris disabled and enters an out of synchronization state so that theinduced voltage waveform does not move from 0.

If the drive current is adjusted so that the induced voltage waveformhas a phase where the drive efficiency is at a maximum, the risk oflosing synchronization is large when the load of the motor fluctuates.Accordingly, although dependent on the actual usage situation of themotor, it is preferable not to perform control to attain a phase wherethe drive efficiency is maximum but preferable to perform control toattain a phase having a slight margin.

Determination Based on Induced Voltage Waveform

FIG. 6 shows examples of induced voltage waveforms in the inducedvoltage detection period. After kickback, state 1 shows a monotonicincrease. This state is considered to have a zero cross positioned nearthe beginning of the detection period. Therefore, the drive current isconsidered to have a slight margin compared to the above-mentionedoptimum (minimum) drive current. Therefore, either this is determined asappropriate or a further detailed determination becomes necessary.Namely, depending on the usage situation of the motor, if the loadfluctuation thereof is comparatively large, the risk of losingsynchronization is large. Thus, since the drive current amount is small,it can be determined that it is necessary to increase the amount.

The drive voltage after kickback in state 2 has a mountain shape. Inthis case, the phase of the induced voltage leads compared with thedrive voltage waveform. Therefore, it is considered to correspond to theexcessive drive current in FIG. 5 and it is determined the currentamount should be decreased.

State 3 has no induced voltage after kickback. Therefore, there is norotation of the rotor and an out of synchronization state can bedetermined.

In the control logic 36 of the drive current adjustment circuit 30, theoutput control circuit 12 is controlled on the basis of thisdetermination result. In the case of state 3, the control logic 36outputs a signal indicating the loss of synchronization was detected.The above-mentioned signal is received by a controller (not shown) forcontrolling the driver circuit 20.

In this manner, the embodiment determines the motor drive state inaccordance with the induced voltage waveform in the induced voltagedetection period and controls the motor drive current. Therefore, thedrive state of the motor is accurately grasped and an appropriate motordrive control can be performed.

The control logic 36 performs determination from the digital data of theinduced voltage. For example, it is preferable to perform theabove-mentioned waveform determination from a comparison of threedetection values. Here, the magnitude of the kickback differs dependingon the magnitude of the coil current. Accordingly, it is preferable toperform the actual detection in the second half of the detection periodto eliminate the influence of kickback as much as possible and detectthe induced voltage waveform. For example, it is preferable to dividethe detection period into eight periods and perform detection at 6/8,7/8, and 8/8 period. At 8/8, it is also possible to detect loss ofsynchronization due to the voltage being 0V.

Estimation of Zero Cross

As described hereinabove, with the induced voltage waveform basicallyhaving a monotonic increase, a target phase is set in the embodiment sothat the zero cross point exists prior to 4/8 of the detection period.Accordingly, in the case where the induced voltage waveform has amonotonic increase, it is preferable to obtain a slope from thedetection values at 6/8 and 8/8, estimate and compare the zero crosswith the target phase, and control the change in drive current. Thiswaveform detection is performed in the control logic 36 of the drivecurrent adjustment circuit 30 and the output control circuit 12 iscontrolled in accordance with the output from the control logic 36.

FIG. 7 shows states of estimating the zero cross point. For example,with regard to the induced voltage waveform in a state of monotonicincrease after kickback, two points are detected. If the time betweenthe two points is ΔT and the difference in induced voltages between thetwo points is ΔV, the slope of the induced voltage waveform is ΔV/ΔT.For example, if the induced voltages at points 6/8 and 8/8 are detectedwhen the above-mentioned detection period has been divided into eightequal parts, ΔT is ¼ of the detection period, and if the induced voltageis V0 at 8/8 with ΔV×4=V0, the point at 0/8 is estimated to be the zerocross point.

In this manner, with regard to the induced voltage, detecting theinduced voltage at the two points separated by set time ΔT enables thezero cross point to be estimated. Then, it is preferable to set a marginfor the motor drive current from the motor load fluctuation, set atarget for the zero cross point, and perform control so that the zerocross point approaches the target phase.

If the estimated zero cross point is delayed in comparison with thetarget phase, the current amount is increased, and if leading, thecurrent amount is decreased. In the case where the difference with thetarget phase is large, the unit amount of the increase or decrease maybe changed. In the case where the difference with the target phase iswithin a predetermined range, an increase or decrease need not beperformed.

Furthermore, the change in the unit amount may be accomplished bychanging the frequency and not changing the unit amount for one time.Namely, if the change of one unit amount is performed twice perdetection, the gain doubles.

In particular, in the case where there is a tendency for insufficientcurrent amount, it is necessary to restore the current amount earlysince there is a risk of loss of synchronization. For example, withrespect to a normal range of control for the drive current, the unitamount (one step) is set to 1/256 and control (change of one unitamount) is performed once for one period of an electrical angle and nearthe loss of synchronization control is performed four times (change offour unit amounts) for one period. In the embodiment, detection isperformed four times during one period (electrical angle of 360°) of themotor so that control can be performed four times, one for everydetection. When changing only once, it is also preferable to performcontrol for increase or decrease only when the same determination resultis obtained four times.

Furthermore, depending on the motor characteristics and magnitude of thedrive voltage, it is necessary to change the control. Accordingly, it ispreferable for the control gain (unit amount) to be variable.

Furthermore, depending on the motor characteristics, there are caseswhere the kickback width increases and the waveform detection of theinduced voltage cannot be performed. In these cases where detection ofthe induced voltage cannot be performed, it is preferable to performdrive at maximum current and to not perform adjustment control of thedrive current.

Furthermore, in the case of an application to systems having lowfluctuations of load on the motor, the terminal for outputting theinduced voltage is set only as OUT1. This can reduce the number of theswitches 32 and the size of the driver 100.

EFFECT

According to the embodiment, high efficiency operation of the motor ispossible. Therefore, efficient motor drive can be performed by reducingthe power consumption. Furthermore, since the drive operation is smooth,the generation of vibration and noise can be suppressed. Moreover, thehigh efficiency operation yields the effects of suppressing thegeneration of heat and simplifying the cooling mechanism.

Furthermore, when detecting the induced voltage, the waveform can bedetected by inputting the voltage directly into the ADC 34 without theneed for obtaining the difference. Thus, op-amps can be omitted and thecircuit can be simplified.

This high efficiency control has maximum effectiveness during normaloperation where the rotation operation is continuous and it ispreferable to perform another control operation, or the drive operationat maximum current, such as at startup. It is recommended that thiscontrol be performed only when the speed of rotation is a predeterminedvalue or higher.

While there has been described what are at present considered to bepreferred embodiments of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

1. A driver circuit for a stepping motor comprising two coils withdissimilar phases of supply currents applied to the two coils androtating a rotor driven by the coils; one terminal of said coils isconnected to a constant voltage and another terminal is set to highimpedance, an induced voltage generated at the coil is detected as avoltage with respect to said constant voltage, and magnitude of motordrive current supplied to said coil is controlled in accordance with astate of detected induced voltage.
 2. A driver circuit according toclaim 1, wherein: one end of said coil is connected to ground and theinduced voltage obtained from another end is directly converted fromanalog to digital.
 3. A driver circuit according to claim 1, wherein:the direction of drive current of said two coils is changed in apredetermined period by switching the polarity of the applied drivevoltage and detection of said induced voltage is performed whenswitching the polarity of the applied drive voltage.
 4. A driver circuitaccording to claim 1, wherein: with a plurality of detection values ofthe induced voltage, the waveform of the induced voltage is detected andthe magnitude of motor drive current supplied to the two coils iscontrolled in accordance with the detected waveform.
 5. A driver circuitaccording to one of claims 1-3, wherein: with a plurality of detectionvalues of the induced voltage, the slope of the induced voltage isdetected, a zero cross point is estimated on the basis of the detectedslope, and the magnitude of motor drive current supplied to the twocoils is controlled in accordance with the phase of the estimated zerocross point.
 6. A driver circuit for a stepping motor comprising twocoils with dissimilar phases of supply currents applied to the two coilsand rotating a rotor driven by the coils; the waveform is detected ofthe induced voltage generated in a phase where the drive current forsaid two coils becomes 0, the suitability of the supply current isdetermined in accordance with the type of detected waveform, and themagnitude of the motor drive current supplied to said coils iscontrolled in accordance with the determination result.
 7. A drivercircuit according to claim 6, wherein: the waveform of said inducedvoltage is determined from at least three voltage values; if following akickback waveform the induced voltage waveform has a monotonic increase,suitability or insufficient motor drive current is determined; and iffollowing a kickback waveform the induced voltage waveform decreasesafter monotonic increase, an excessive motor drive current isdetermined.
 8. A driver circuit according to claim 7, wherein: if theinduced voltage waveform has a monotonic increase, a zero cross point ofthe induced voltage waveform is determined on the basis of the inducedvoltage waveform and the magnitude of the motor drive current suppliedto the two coils is controlled in accordance with the phase of theestimated zero cross point.
 9. A driver circuit for a stepping motorcomprising two coils with dissimilar phases of supply currents appliedto the two coils and rotating a rotor driven by the coils; the slope ofan induced voltage generated in a phase where the drive current for saidtwo coils becomes 0 is detected; a zero cross point of the inducedvoltage waveform is estimated from the detected slope, and the magnitudeof motor drive current supplied to the two coils is controlled inaccordance with the phase of the estimated zero cross point.
 10. Adriver circuit according to claim 9, wherein: the slope of said inducedvoltage is detected during monotonic increase following kickbackwaveform of the induced voltage with said coils set in a high impedancestate.